This invention relates to plasma or reactive ion etch processes for preferentially etching silicon dioxide over silicon in integrated circuit manufacture. Specifically it relates to the use of a plasma etch gas incorporating BF.sub.3 and, preferably, BF.sub.3 in combination with a formaldehyde species for selectively etching thin layers of silicon dioxide over silicon.
An important step in the manufacture of semiconductor chips and thin film circuitry is the etching of the different layers such as polysilicon and silicon dioxide which make up the finished semiconductor chip or the thin film circuit. In the manufacture of these thin film circuits, one method of etching has been to overlay the surface to be etched with a suitable mask and to immerse the circuit so masked in a chemical solution which attacks the surface to be etched while leaving the mask otherwise intact. It has been difficult with the chemical etching processes presently known to achieve well-defined edges on the etched surfaces. The difficulty arises because the chemical action tends to etch isotropically, i.e., it undercuts the mask by the same distance that it penetrates the underlayer, and thus enlarges the feature. It is, therefore, very difficult to use wet chemical etching to achieve fine structures; fine structures being defined as structures having geometries on the order of one micron.
One known method for manufacturing semiconductor chips is called "plasma etching" in which a vacuum chamber containing an excited and a grounded electrode is filled with a gas such as a fluorocarbon or SF.sub.6 at low pressure. The electric field established between the electrodes serves to dissociate the gas forming a plasma capable of etching a SiO.sub.2 surface.
The ability to etch thin (approximately 1.mu.) layers of SiO.sub.2 over Si is an important aspect of the plasma etch processes used in microcircuit manufacture. The plasma field of any given apparatus or oxide layer thickness is not necessarily uniform over the entire wafer surface, thus the etch may penetrate the SiO.sub.2 layer at different times. In order to avoid etching the underlying silicon underlayer where the oxide has first cleared, it is desirable to arrange a selective plasma process which does not seriously affect or diminish the etch rate on SiO.sub.2 but significantly decreases the rate on silicon.
Plasma etching wherein various plasma generating compositions, such as CF.sub.4, CHF.sub.3, C.sub.2 F.sub.6, C.sub.2 F.sub.4, CCl.sub.4, BCl.sub.3, etc., including admixtures, with O.sub.2, N.sub.2, Ar or air, are contained in a reactor under a specific pressure are known in the prior art. The material to be etched is introduced into the reactor and the etching process is effected by radicals formed by the plasma. In the past, such compositions have not yielded consistently satisfactory results. It would accordingly be desirable to have a plasma gas composition which when used with low frequency electrical excitation would produce selective etching of thin layers of silicon dioxide over silicon.
The present invention utilizes a BF.sub.3 based composition in plasma etching of silicon dioxide preferentially over silicon or polysilicon and while the prior art describes the use of BF.sub.3 among other gases that may be used in an ion etching process, as disclosed in Japanese Patent Publication No. 137,636 of Oct. 27, 1981, no prior art is known which discloses the unique advantages of BF.sub.3 plasma compositions in the selective etching of silicon dioxide over silicon or polysilicon.